The present invention relates to an amplifier whose transmission factor, i.e. gain or attenuation factor, can be controlled by means of a control voltage and which can be switched between two operating states having complementary control characteristics.
Such amplifiers are required, for example, in signal amplitude compression-expansion, or compander, systems for noise suppression as they are disclosed in German Offenlegungsschrift [Laid-Open Application] No. 22 18 823, as well as in German Auslegeschrift [Published Application] No. 24 06 258 and corresponding U.S. application Ser. No. 543,931, filed Jan. 24th, 1975. These systems require an amplifier which during dynamic compression has a certain control characteristic corresponding to a certain dependence of the amplitude of its output signals or its gain, on a control voltage, and during dynamic expansion has a control characteristic corresponding to a complementary dependence.
German Offenlegungsschrift No. 22 18 823 describes an amplifier which can be switched between two operating states and in which a variable gain is effected by means of a controllable impedance which, in the one operating state of the amplifier, is connected to a feedback path of the amplifier and, in the other operating state, is connected to the input of the amplifier.
A drawback inherently present in simple compander systems, i.e. modulation of the noise components in dependence on low frequency useful signals, has led to the requirement, described in Funktechnik, 1973, pages 55-57, particularly FIG. 6, for a shift of the lower frequency limit of the circuit effecting the dynamic compression or expansion, in dependence on the amplitude of the useful signals, toward higher frequency values, while keeping the resulting characteristics as congruent as possible.
In the amplifier disclosed in German Offenlegungsschrift No. 22 18 823, whose control input receives a control voltage derived from the useful signal, a capacitance is connected in series with the controllable impedance so that a slight dependence on the amplitude of the useful signals results for the lower frequency limit of the amplifier. However, the shift of the lower frequency limit obtained in this way is limited to a narrow range. Moreover, in the known circuit the amplitude of the amplifier output signals in the transmitting range is also changed in an undesirable manner.